In the proceeding of low power consumption of a power conversion apparatus, low power consumption of a power device performing an important function in the power conversion apparatus has been demanded. Particularly, since an insulated gate transistor (MOSFET) or an insulated gate bipolar transistor (IGBT) of which gate can be driven by a voltage can be easily treated, application fields have been greatly widened.
The MOSFET and the IGBT have a built-in parasitic structure. In other words, the MOSFET is a parasitic bipolar transistor (NPN structure), and the IGBT is a parasitic thyristor (PNPN structure). Particularly, in the case of the IGBT, since the IGBT includes a parasitic thyristor, if the parasitic thyristor operates, although electron injection from an inversion layer of the MOS gate is stopped by setting the gate voltage to a threshold value or less, the electrons continues to be injected through other paths from an n-type source layer into a p-type base layer. This phenomenon is called latch-up. When the gate is in a turned-on state or at a turned-off time, if the latch-up occurs, the controllability of the gate voltage is lost, and in the worst case, the device may be destructed.
As measures to suppress the destruction caused by the latch-up, there is a method of forming a p-type contact layer having a concentration higher than that of a p-type base layer in an inner portion of the p-type base layer constituting the MOS gate. FIG. 26 is a cross-sectional view for describing operations of a semiconductor device in the related art. FIG. 26 illustrates cross sections illustrating only the extracted portion of the gate structure of the IGBT or the MOSFET. FIG. 26(a) illustrates a cross section in the case where only the p-type base layer 64 is a p-type layer. FIG. 26(b) illustrates a cross section in the case where a p-type contact layer 66 having a concentration higher than that of a p-type base layer 64 is formed on a surface of the p-type base layer 64.
In FIG. 26(a), in a turned-on state or at a turned-off time, holes flowing into the p-type base layer 64 pass through a region between the n-type source layers 65 to an emitter electrode (not illustrated) as a hole flow 17. Although the hole flow 17 illustrated as a rectangular shape in FIG. 26(a) for the convenience of illustration, actual holes flow in a curve according to acceptor concentration distribution or electrostatic potential distribution.
In this manner, when the holes flow in an inner portion of the p-type base layer 64, large voltage drop occurs due to a resistance component 16 of the p-type base layer 64. If the voltage drop is larger than a built-in potential of pn junction between the n-type source layer 65 and the p-type base layer 64, a forward-biased voltage is generated at the pn junction, so that electrons are injected into the p-type base layer 64 through a path different from the MOS gate. As a result, electron injection at the MOS gate may not be controlled. In FIG. 26, an emitter electrode is denoted by reference numeral 72.
On the other hand, in FIG. 26(b), since the p-type contact layer 66 having a concentration higher than that of the p-type base layer 64 is formed in an inner portion of the p-type base layer 64, the magnitude of the resistance component 16 in the inner portion of the p-type base layer 64 is decreased due to the p-type contact layer 66. Therefore, even in the case where a larger current flows, the voltage drop due to the current can be suppressed to be equal to or lower than the built-in potential of the pn junction between the n-type source layer 65 and the p-type base layer 64.
As a result, it is possible to prevent a parasitic thyristor or a parasitic bipolar transistor from operating. In FIG. 26(b), an interlayer insulating film is denoted by reference numeral 9; a gate oxide film is denoted by reference numeral 10; a polysilicon electrode is denoted by reference numeral 11; and an n-type drift layer is denoted by reference numeral 61.
In addition, in the related art, there is a structure of forming a p-type high concentration layer in a lamination shape which is formed in an inner portion of the p-type base layer 64 to be in contact with the p-type contact layer 66 in order to prevent the parasitic thyristor or the parasitic bipolar transistor from operating (for example, refer to patent Document 1 listed below). FIG. 25 is a cross-sectional view illustrating main components of a semiconductor device in the related art. FIG. 25 illustrates a cross section of a planar gate IGBT where the p-type high concentration layer is formed.
As illustrated in FIG. 25, in addition to the p-type contact layer 66, a p-type high concentration layer 28 is formed in an inner portion of the p-type base layer 64 in a lamination shape so as to be separated from the n-type source layer 65, so that resistance distribution according to carrier transport is alleviated. In addition, patent Document 1 discloses a method of forming the p-type high concentration layer 28 by a high acceleration voltage ion implantation method and thermal treatment.
On the other hand, in the related art, there are a technique of a trench gate-type IGBT including a p-type high concentration layer 28 which is formed in an inner portion of a p-type base layer to be in contact with a p-type contact layer (for example, refer to FIG. 4 in patent Document 2 listed below) and a technique of an IGBT including a deep p-type well layer 26 in an inner portion of a p-type base layer 64 (for example, refer to patent Document 3 listed below). In FIGS. 25 and 27, an n-type field stop layer is denoted by reference numeral 2; a p-type collector layer is denoted by reference numeral 3; and a collector electrode is denoted by reference numeral 13.
FIG. 27 is a cross-sectional view illustrating main components of a semiconductor device in the related art. FIG. 27 illustrates a cross section of the aforementioned IGBT. A deep p-type well layer 26 is installed in an inner portion of the p-type base layer 64 including the p-type contact layer 66. In FIG. 27, the deep p-type well layer 26 also has the same function as the aforementioned p-type high concentration layer 28, so that the effect of reducing the resistance component of the path through which the holes flow can be obtained.